Heat sink

ABSTRACT

A heat sink comprising: a body having an envelope including a flexible portion for thermal contact with a heat source, the envelope being filled with a thermally conductive liquid; and a thermosyphon having one end disposed at least partly within the liquid in the body to absorb heat from the heat source and another end disposed outside the envelope to dissipate heat.

This application is a divisional of U.S patent application Ser. No.08/996,014, filed Dec. 22, 1997, now U.S. Pat. No. 6,062,299.

THIS INVENTION relates to a heat sink and more particularly to a heatsink for cooling a heat source such as electronic components mounted ona printed circuit board.

The trend in the development of electronic devices is for heatgenerating components to be assembled in high packaging densities onprinted circuit boards (PCBs). Therefore, the heat to be dissipated atboth component and PCB levels has increased tremendously andconventional free or forced air convection cooling techniques havereached their maximum cooling capacity. Since the external surfaces ofheat generating components mounted on a PCB are not always in the sameplane, cooling all the components on a PCB by a single solid heat sink,such as a heat pipe, is very difficult.

Heat sinks with flexible surfaces have been proposed. The main objectivefor such heat sinks is to cool individual electronic components. Heatsinks incorporating flexible containers for cooling multiple electroniccomponents have also been proposed. However, the cooling capacities ofsuch designs are limited since no effective method has been proposed fordissipating the heat absorbed by a liquid inside the flexible container.

It is an object of the present invention to provide an improved methodfor cooling heat generating electronic components mounted on PCBs.

Accordingly, the present invention provides a heat sink comprising: abody having an envelope including a flexible portion for thermal contactwith a heat source, the envelope being filled with a thermallyconductive liquid; and a thermosyphon having one end disposed at leastpartly within the liquid in the body to absorb heat from the heat sourceand another end disposed outside the envelope to dissipate heat.

In order that the present invention may be more readily understood,embodiments thereof will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a heat sink embodying the presentinvention, a cut-away portion illustrating the interior of a section ofthe heat sink;

FIG. 2 is a schematic side view, partially in cross-section of the heatsink of FIG. 1, two PCBs being shown in phantom either side of the heatsink;

FIG. 3 is a schematic side view of another heat sink embodying thepresent invention, a single PCB being shown in phantom on one side ofthe heat sink;

FIG. 4 is a schematic side view of a further heat sink embodying thepresent invention for use in cooling two PCBs; and

FIG. 5 is a schematic side view of another heat sink embodying thepresent invention for use in cooling a single PCB.

Referring to the figures, a heat sink 1 embodying the present inventionhas a body 2 for engagement with heat generating components on at leastone printed circuit board, a series of thermosyphons, or heat pipes, 3and an array of cooling fins 4 mounted on the thermosyphons 3.

Referring to FIGS. 1 and 2, the body 2 comprises a substantiallyrectangular frame 5 made up of side walls 6, 7, a top wall 8 and a base9. Two flexible membranes 10, 11 are attached to the frame 5 to create avolume within the body 2 which is enclosed by the side walls 6, 7, thetop wall 8, the base 9 and the flexible membranes 10, 11. The flexiblemembranes 10, 11 are made from a flexible material having high thermalconductivity such as, for example, laminated aluminum foil or a 3Mproprietary multi-layer film as used in 3M's liquid heat sink.

The top wall 8, is formed with a series of equispaced holes along itslength each of which receives in friction fit and sealing engagement athermosyphon 3. The end of the thermosyphon 3, inside the body 2, isknown as the evaporator portion 12. The evaporator portion 12 generallyestablishes a plane that bisects the body 2. The opposite end of thethermosyphon 3, which is located outside of the body 2 is known as thecondenser portion 13. Each thermosyphon 3 is filled with a working fluidsuch as water.

The top wall 8 of the frame 5, is also provided with a pair of valves14, 15, which are used to charge and pressurize a thermally conductiveand electrically non-conductive liquid which can be introduced into thebody 2. Valves 14 and 15 are perferably positioned on opposite sides ofthe plane of evaporator portion 12 that generally bisects the body 2.Preferably, the liquid in the body is 3M's Fluorinert (Trade Mark)liquid.

One of the valves 14 is used to bleed air or the like from the body 2 asthe body 2 is being filled with liquid through the other of the valves15. The condenser portions 13 of the thermosyphons 3 support the arrayof cooling fins 4. Each fin 4 comprises a rectangular plate manufacturedfrom a thermally conductive material such as copper. The fins 4 arespaced apart from one another to maximize the surface area which theypresent to promote efficient cooling.

In operation, the body 2 of a heat sink 1 embodying the presentinvention is sandwiched between two PCBs upon which are arranged anarray of heat generating components requiring cooling. As shown in FIG.2, the components on the PCBs face inwardly towards the body 2 of theheat sink 1.

The volume inside the body 2 is filled through one of the valves 14, 15with the thermally conductive liquid. When filled, the liquid in thebody 2 is then pressurized through one of the valves 14, 15 so that theflexible membranes 10, 11 expand to make good thermally conductivecontact with the irregular surfaces presented by the electroniccomponents on both the PCBs. In this manner, the maximum surface area ofthe electronic components on the PCBs is in contact with the surfaces ofthe thermally conductive membranes 10, 11 even though the components onthe PCB are not necessarily all in the same plane. Preferably, theflexible membranes 10, 11 are coated with a thermally conductive gel toenhance the thermal conductivity between the components and themembranes 10, 11.

As the PCB components heat up, heat is conducted through the membranes10, 11 to the liquid inside the body 2 which absorbs this heat andtransmits the heat to the evaporator portions 12 of the thermosyphons 3.In forced convection embodiments of the invention, as shown in FIGS. 1to 3 and in free convention embodiments of the invention as shown inFIGS. 4 and 5, as the evaporator portions of the thermosyphons 3 heatup, evaporation of the working fluid inside the thermosyphons 3 iscaused, causing the vapour to move up the thermosyphons 3 towards thecondenser portions 13. As the vapour reaches the condenser portions 13of the thermosyphons 3, the heat of the working fluid is conducted tothe fins 4. In both forced and free convection embodiments, the heatfrom the condenser portions 13 is radiated outwardly via the coolingfins 4 to the atmosphere by forced air convection or free convection.

A variation on the embodiment of the heat sink shown in FIGS. 1 and 2 isshown in FIG. 3. In this embodiment, one of the flexible membranes 11,is removed from the body 2 and substantially planar rigid metal plate16, that is opposite the remaining flexible membrane 10 and transverseto top wall 8. This embodiment is particularly well suited to coolingthe components of a single PCB, the components of the PCB being placedin contact with the remaining flexible membrane 10.

The embodiments of the heat sink shown in FIGS. 1, 2 and 3 areparticularly well suited to cooling by forced air convection whereas theembodiments shown in FIGS. 4 and 5 are particularly well suited tocooling by free convection. In the embodiments shown in FIGS. 4 and 5,the thermosyphons 3 are bent through 85° and the fins 4 are mountedvertically, i.e. parallel with the evaporator portions 12, within thebody 2 so that heat is dissipated from the fins freely. The embodimentshown in FIG. 4 is particularly well suited for cooling two printedcircuit boards, whereas the embodiment shown in FIG. 5 is particularlywell suited for cooling a single PCB, one of the flexible membranes 11being removed and replaced by a rigid metal plate 16.

When the heat sink body 2 is to be removed from between the PCBs, thebody is simply depressurized by opening one of the valves 14, 15 tobleed off air and/or liquid to allow the flexible membrane or membranes10, 11 to collapse away from the PCB(s).

It will be obvious to those having skill in the art that various changesmay be made in the details of the above-described embodiments of thepresent invention without departing from the principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

What is claimed is:
 1. A heat sink comprising: a body having an envelopeincluding two flexible portions for thermal contact with a heat sourcelocated on opposite sides of the envelope and an upper surface that isgenerally transverse to the flexible portions, the envelope being filledwith a thermally conductive liquid; a thermosyphon having multipleevaporator portions disposed at least partly within the liquid in thebody to absorb heat from the heat source and multiple condenser portionsdisposed outside the envelope and extending from the upper surface todissipate heat; a first valve located on the body through which theliquid is charged into the envelope to come into contact with all of theevaporator portions; and a second valve located on the body throughwhich vapor is discharged out of the envelope and away from all of theevaporator portions, such that the envelope is capable of being filledwith liquid through the first valve while the vapor is eliminatedthrough the second valve.
 2. A heat sink according to claim 1, whereinat least one of the flexible portions is made from a thermallyconductive material.
 3. A heat sink according to claim 1, wherein atleast one of the flexible portions comprises a flexible membrane.
 4. Aheat sink according to claim 3, wherein at least one of the flexibleportions is mounted on a frame.
 5. A heat sink according to claim 4,wherein the frame and at least one of the flexible portions areelectrically non-conductive.
 6. A heat sink according to claim 1,wherein a plurality of thermosyphons are provided.
 7. A heat sinkaccording to claim 1, wherein a cooling fin is attached to thethermosyphon outside the envelope.
 8. A heat sink according to claim 1,wherein at least one of the flexible portions is positioned againstmultiple electrical components on a PCB, the multiple electricalcomponents having irregular surfaces in different planes.
 9. A heat sinkaccording to claim 1, wherein the first or second valve is disposedoutside the envelope above the upper surface.
 10. A heat sink accordingto claim 1, wherein the evaporator portions generally bisect the body.11. A heat sink according to claim 1, wherein the evaporator portionsgenerally bisect the body and wherein the first and second valves aredisposed outside the envelope, extending from the upper surface, and onopposite sides of the evaporator portions.
 12. A heat sink according toclaim 1, wherein the first and second valves are separately connected tothe body.
 13. A heat sink according to claim 1, wherein the thermosyphonis adapted to contain a working fluid that is separated from thethermally conductive liquid in the body.
 14. A heat sink according toclaim 1, wherein the envelope is pressurized through the first valve anddepressurized through the second valve.
 15. A heat sink according toclaim 1, wherein all of the vapor is bled from the envelope when theenvelope is filled with liquid.
 16. A heat sink according to claim 2,wherein at least one of the flexible portions comprises a flexiblemembrane.
 17. A heat sink according to claim 5, wherein the evaporatorportions generally bisect the body.
 18. A heat sink according to claim5, wherein the evaporator portions generally bisect the body and whereinthe first and second valves are disposed outside the envelope, extendingfrom the upper surface, and on opposite sides of the evaporatorportions.
 19. A heat sink according to claim 13, wherein the first andsecond valves are separately connected to the body.
 20. A heat sinkaccording to claim 11, wherein the thermosyphon is adapted to contain aworking fluid that is separated from the thermally conductive liquid inthe body.